How do you troll

Turning out the lights in the toilets when someone is taking a shit is a good wheeze. (needs bogs with no windows, of course) Wait a minute or two until they're shuffling out of the cubicle holding their pants at half mast to turn the light back on, then barge in and stare at them in horror and revulsion.

Something like this happened to a mate of mine. The lights got turned out by a mental caretaker and it ended up in a fight. My friend got a beating because he's a soft shite.